In a video tape recorder (VTR), for example, standard external synchronization signals are used and image signals are reproduced based on these external synchronization signals. That is, in a VTR, speech signals transiently stored in a memory are reproduced to follow up with clocks generated on the basis of external synchronization signals (reference clocks).
The input data volume and the output data volume to or from the memory occasionally undergo local deviation from the reference clocks. However, these data volumes are generally constant. Thus, in the above VTR, the writing clocks (input side clocks) and the readout clocks (output side clocks) are controlled at a pre-set velocity so that partial offset from the reference clocks is taken up by a time base corrector (TBC).
However, in a system devoid of the reference clocks, for example, in a projector, the image signal playback speed is determined by the film frame feed speed, such that there is no guarantee for an absolutely constant playback speed, with the playback speed being varied both locally and comprehensively. The speech signals need to be reproduced in synchronism with this playback speed of the image signals. However, with the conventional system, such as a projector, the output side clock rate is perpetually constant without regard to how much the input side clocks of the memory in which the speech signals are transiently stored, that is the clocks based on the playback speed of the image signals, deviates from the output side clocks. The result is that deviation is produced between the image and the speech and that, if the input side clocks are monotonously increased, the memory is occupied to its maximum capacity such that memory rupture occurs.
If, for preventing deviation between the image and the speech or evading the memory-full state, the memory input side clocks are compulsorily matched to the memory output side clocks by, for example, a phase-locked loop (PLL), the output side clocks react hypersensitively to local fluctuations of the input side clocks, thus outputting wow and flutter. If, for prohibiting the outputting of the wow and flutter, the reaction response speed of the output side clocks is delayed, prolonged time elapses until the output side clock reaction subsides to a steady state, thus again producing deviation between the image and the speech.
That is, with the conventional memory control, there are only two speeds at which the data is outputted from the memory (output speeds), that is a speed for a steady-state mode and a speed for an accelerated mode. The result is that the output side clocks follow up at a constant speed, for example, a speed added to with a speed increment s1, without regard to the extent of offset between the output side clocks and the input side clocks.
If the current output speed deviates significantly from the target speed, a prolonged time t.sub.d is required until the output speed catches up with the target speed S.sub.d, as shown in FIG. 4 showing memory control contrasted to that in the synchronous control device of the present invention, as later explained, such that, during this time t.sub.d, the speech remains deviated from the image.
Moreover, if the current output speed is slightly deviated from the target speed, the output speed reaches the target speed S.sub.d in a shorter time t.sub.d as shown in FIG. 5 showing memory control contrasted to that in the synchronous control device of the present invention as later explained, that is the output speed is changed abruptly, thus producing a non-spontaneous sound.
It is therefore an object of the present invention to provide a synchronous control device whereby the output side data rate can be reliably synchronized with the input side data rate.
It is another object of the present invention to provide a synchronous control device whereby the speech can be reliably synchronized with the image.